The present invention relates to an apparatus for detecting a rotational position, and more particularly, to an apparatus for detecting the position of rotation of an object by correcting errors caused by eccentricity of the object.
More particularly, the present invention relates to an apparatus for detecting a rotational position, and more particularly to an apparatus which detects the rotational position of a rotating object such as a spindle and a motor shaft in a motor which actuates the spindle or feed shafts in a machine tool and in a controller therefor.
In this controller, a feed back control is generally used so that a rotational position information on the spindle and the motor shaft is fed back to the controller. Therefore, the machining accuracy of the machine tool depends on the accuracy of the position detecting apparatus. In this connection, there is a requirement of a high accuracy positional detection.
FIG. 1 shows the structure of prior art gear-type magnetic detecting apparatus for detecting a rotational position. In the figure, two-phase AC signals V sin .theta. and V cos .theta. having phase difference of 90.degree. from each other are generated from a two-phase AC signal generating device 20, which includes a gear 1, a permanent magnet 2 and magnetic sensors 3 and 4. A signal generating device 13 includes sample-and-hold (S/H) circuits 11a and 11b and analog/digital (A/D) converters 12a and 12b, and extracts from the two-phase AC signals two-phase instantaneous signals and outputs the same. A higher-order signal generating device 14 includes comparators, a pulse counter, etc., has input thereto the two-phase AC signals, and counts the number of waves of the two-phase AC signals. A positional signal generating device 17 includes a microcomputer and the like and executes an operation, .theta.=tan.sup.-1 (V sin .theta./V cos .theta.) based on the two-phase instantaneous signals and adds the number of waves from the higher-order signal generating device 14 to detect a position .alpha. within one rotation of the gear 1.
In the prior art detecting apparatus for detecting the rotational position as mentioned above, however, it is difficult to precisely align the center of the gear with that of the rotary shaft. In practice, so-called eccentricity, or displacement of the center of the gear from the center of the rotary shaft, is found to occur, and this eccentricity causes of microdisplacement in the direction of the gear rotation corresponding to the rotational angle. The operation mentioned above is not therefore sufficient to determine the precise position of rotation.